The actions of acetylcholine include a decrease in heart rate, force of contraction and conduction through the atrioventricular node. The cardiac effects of muscarinic agonist are often attributable to changes in transmembrane ionic currents. Although we have some understanding of how cardiac muscarinic receptors are coupled to the regulation of ionic currents our knowledge is incomplete. Better understanding of the physiological mechanisms involved in signal transduction will provide a greater potential for designing therapeutic interventions. The mechanisms involved in coupling muscarinic receptors to L-type calcium current, the acetylcholine-induced potassium current and the pacemaker current will be studied. In the ventricle, prior elevation of intracellular cyclic AMP levels is required for muscarinic inhibition of calcium current to occur. Mechanistic studies using the whole cell patch clamp method are consistent with the notion that inhibition of adenylate cyclase activity contributes to muscarinic regulation of ventricular calcium current. Evidence also exists for cyclic AMP-independent pathways for regulation of calcium current in the ventricle, in particular cyclic GMP-dependent pathways. The pipet solutions that are usually used in whole cell patch clamp method would prevent activation of guanylate cyclase and might therefore have blocked important metabolic events. One portion of the studies proposed will reexamine the mechanism for muscarinic regulation of ventricular calcium current by comparing results using a less invasive recording method, the perforated patch technique, to those obtained using the standard whole cell patch clamp method. Muscarinic regulation of the calcium current will be examined in the presence of isoproterenol, forskolin, isobutylmethylxanthine and membrane permeant cyclic AMP derivatives. Because modulation of cardiac L-type calcium current occurs in the absence of beta-adrenoceptor stimulation in mammalian atrium, different pathways might be involved in the regulation of atrial calcium current. The pathways involved in atrial receptor-effector coupling to calcium current will also be examined. Muscarinic receptor stimulation activates a potassium current in atrial cells. Evidence suggests that a GTP-binding protein interacts directly with the channel responsible for' this current and thereby activates the current. The identity of the GTP-binding protein involved in the physiological coupling to the receptor is unknown. Studies are proposed that will make use of antisense oligonucleotide to identify the GTP-binding protein responsible for potassium channel activation. Oligodeoxyribonucleotides that are complementary to the mRNA for specific GTP-binding proteins will be introduced into the cells. This intervention is capable of inhibiting the production of specific proteins. By comparing the effects of oligonucleotides directed against specific GTP-binding proteins it should be possible to identify the one that is responsible for the physiological coupling. Cyclic AMP is involved in the regulation of the pacemaker current. As is the case for calcium currents, there is evidence that cyclic AMP-independent pathways also exist. This will be investigated using both standard whole cell and perforated patch recording from sinoatrial node cells.